Could the earthquakes in the Muswellbrook area be caused by mining? Geologists explain

On Friday 23 August, a 4.8 magnitude earthquake struck New South Wales near Muswellbrook. The quake caused localised damage, and the Australian Geoscience Service received more than 3,600 reports of the quake, including from Sydney and Canberra, 360 kilometres away.

Since then, there have been several aftershocks, including one measuring 4.6 on the Richter scale over the weekend. The series of quakes occurred directly beneath the operating Mount Arthur coal mine, so it’s clear that these quakes are linked to local mining. But correlation doesn’t mean causation. Let’s take a look at what we know.

What are the causes of earthquakes?

Earthquakes typically occur when stresses build up in the Earth's crust as a result of tectonic forces. Once enough stress has built up, pre-existing weak areas or “faults” in the crust slip – that's an earthquake. The stresses are released in the form of seismic energy waves.

Human activities that cause changes in stress on the Earth's crust can also cause earthquakes. These are called “anthropogenic” earthquakes: without human activity, these earthquakes would not occur.

“Triggered” earthquakes occur on existing fault structures, but their timing is brought forward by human activity: they would have happened anyway, but the stresses introduced have made them happen a little faster.

Human activities that can lead to induced and triggered earthquakes include hydraulic fracturing, wastewater injection, filling of man-made reservoirs, and mining.

In open-pit coal mining, removing large amounts of rock from the surface can change stresses locally and may trigger earthquakes.

At the Mount Arthur coal mine, seismic monitoring has been ongoing since the 1990s. While a smaller mine operated before then, satellite images on Google Earth show a major expansion—more than double the size of the original mine—beginning in 2002.

Seismic activity in the area has increased since about 2014. This appears to indicate that the crust was responding to stress changes due to mining at the site.

Seismic activity in the Muswellbrook area. From left to right: 1994–2004, 2004–2014, 2014–present. Adam Pascal/Earthquake Research Centre What has happened recently in Muswellbrook?

Since Friday's 4.8 magnitude quake, there have been 20 more with magnitudes greater than 2.5. All of those quakes were within 5 kilometers of the surface. That's considered “shallow” and could indicate that the quakes happened because the removal of coal from the surface changed the stress in the crust.

So the shallow depth and the likely increased seismic activity since the mine expansion might suggest that the recent earthquakes in the Muswellbrook area were mining-related. But there is also evidence to suggest otherwise.

A recent study suggests that even large open-pit mines do not significantly change the stress along faults near the surface (within 5 km) to trigger moderate-sized earthquakes. Calculations show that faults would already have to be tectonically stressed to near failure for mining operations to affect the timing of earthquakes.

However, shallow-seismic induced earthquakes can occur. These are usually very small—less than 2 on the Richter scale—and most occur within a few hundred metres of the mine.

The area was plagued by earthquakes before mining.

Some earthquakes may occur in the Muswellbrook area due to local geology alone.

The Muswellbrook area lies on the eastern edge of the Sydney Basin, which contains sediments dating back between 200 million and 300 million years. These sediments include organic material that has been transformed into the coal that is now being mined. The eastern edge of this basin is the Hunter-Mookie Fault, a complex fault system that extends for more than 400 kilometres.

Surface geology of the Muswellbrook area, showing the Hunter-Mookie fault. Rasmus et al., 1969: Singleton 1:250 000 Geological Sheet SI/56-01, 1st Edition

To the north of Muswellbrook, an associated fault appears to show evidence of earthquakes in the recent geological past, long before any mining in the area.

The southern limit of the Hunter-Mookie Fault is near Newcastle. Australia's most powerful earthquake ever recorded occurred here in 1989, killing 13 people. It had a magnitude of 5.4.

If some parts of this fault system have produced earthquakes in the past, it is possible that other faults along this boundary could produce earthquakes as well, without any associated mining.

Earthquake clusters are not uncommon in Australia. Between 1886 and 1949, four moderate earthquakes ranging in magnitude from 5.3 to 5.6 occurred in Dalton-Gunning, New South Wales, as part of a long earthquake series. The area continues to experience earthquakes to this day. This and other earthquake series are not located near mining sites.

In general, the evidence available for recent earthquakes near Muswellbrook does not allow us to say unequivocally whether they are related to mining.

Earthquakes can occur anywhere in Australia. Although mining can stimulate seismic activity locally, most of these earthquakes are minor and rarely felt by people. Tectonic forces are still the main cause of moderate earthquakes in Australia.

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